This invention relates to an optical waveguide type device for use as an optical modulator or switch in various optical systems, such as a high-speed optical communication, an optical switching network, an optical information processing and an optical image processing.
Optical waveguide type devices such as optical waveguide modulators and switches are the most important key elements for realizing a high-speed optical communication, an optical switching network, an optical information processing and an optical image processing. The waveguide type optical device utilizes a electro-optical effect having a high operation speed, since the device is required to operate at a super high-speed and a low driving power. The electro-optical effect means such a phenomenon that the index of refraction of a substance is changed by applying an electrical field for the substance. Heretofore, various waveguide type optical devices have been developed by the use of the above electro-optical effect. For example, the above optical waveguide modulator comprises an optical waveguide formed in a substrate having the electro-optical effect and a signal electrode formed on the substrate via a buffer layer. A LiNbO.sub.3 substrate or a GaAs substrate is used as the substrate in many cases. Where the LiNbO.sub.3 is used as the substrate, the optical waveguide is formed by thermally diffusing titanium (Ti) into the LiNbO.sub.3 substrate. This diffusion method of titanium provides a convenient and simple method of fabricating a low-loss strip waveguide having excellent electro-optical properties.
Important parameters of the optical waveguide modulator are a driving power (a driving voltage), a modulation bandwidth, a characteristic impedance and an insertion loss. In this case, the driving power and the modulation bandwidth are in a trade-off relationship. Namely, it is difficult to satisfy both a wide modulation bandwidth and a low driving power at the same time. Therefore, researches of the optical waveguide modulator has been concentrated on optimizing the trade-off relationship.
The modulation bandwidth of the optical waveguide modulator is limited by a microwave attenuation and a velocity mismatch between an optical wave and a microwave. In addition, the modulation bandwidth depends on a type, a material and an arrangement of an electrode and a dielectric constant of the substrate. For broadband applications, a travel-wave electrode is widely used. The concept of the traveling-wave electrodes is that the traveling-wave electrodes are made extensions of a driving transmission line. Thus, the electrode should have a characteristic impedance which is equal to that of a power source and a cable. In this event, a modulation speed is limited by a difference in transit time (a phase velocity or an effective index) between the optical wave and the microwave. The travelling wave electrode has two types of structures, namely, an ASL (Asymmetric Strip Line) or ACPS (Asymmetric Coplanar Strip) electrode structure and a CPW (Coplanar Waveguide) electrode structure.
The microwave attenuation is very important in the sense that, even if a perfect velocity matching is obtained, the bandwidth is limited by the microwave attenuation. Hence, reduction of the microwave attenuation is needed.
The microwave attenuation is caused by,
(a) Stripline conductor loss, which is a function of the electrode arrangement, resistivity of the electrode material and a parameter of the buffer layer etc., PA0 (b) Dielectric loss, which is a function of the dielectric constant and tan.delta. (loss tangent) of the LiNbO.sub.3 substrate etc., PA0 (c) Loss due to higher order mode propagation, PA0 (d) Loss due to stripline bends and tapers, PA0 (e) Loss due to impedance mismatch with a 50.OMEGA. source and a load, PA0 (f) Loss due to a mounting package including loss at a connector, a connector-stripline (signal electrode) contact and an outside package.
Inventors have tried to reducing the microwave attenuation by reducing the above mentioned factors, especially concentrating on the stripline conductor loss and the higher order mode propagation. The inventors have achieved an optical modulator having a wide bandwidth and a relatively low driving voltage by using a thick buffer layer and a thick electrode structure. This is given in paper "A wide-band Ti:LiNb.sub.3 optical modulator with a conventional coplanar waveguide type electrode", IEEE Photonics Technology Letters, vol.4, No.9, pp. 1020-022, 1992. The inventors have achieved a bandwidth of 20 GHz and a driving voltage of 5 V for an electrode length of 2.5 cm. Herein, the higher order mode propagation loss is reduced by decreasing the chip thickness to about 0.2 mm.
In addition, a fiber/fiber connecter having a proper package must be attached to an edge of the chip in a practical use. The chip thickness (or the surface area) at the edge should be the order of few millimeter at a cross section to facilitate the fiber connection. Hence, the chip is usually arranged on a dielectric substrate such as a glass substrate. In this case, the dielectric substrate has thermal characteristics (thermal expansion etc.) compatible with that of the LiNbO.sub.3 substrate to obtain a desired thickness at the edge in the practical device. This dielectric substrate also helps in providing strength to the thin LiNbO.sub.3 chip (the thickness of about 0.2 mm). Further, the chip and the dielectric substrate may be placed in a metal package to obtain both stability and reliability of the operation.
Subsequently, the conventional optical waveguide modulator will be explained in more detail.
First, an optical waveguide is formed in a substrate such as a LiNbO.sub.3 substrate. The substrate is arranged on a dielectric substrate such as a glass substrate. Further, the dielectric substrate is disposed on a metal substrate. The optical waveguide is formed by thermally diffusing titanium (Ti) into the substrate. Subsequently, a buffer layer is entirely formed on the substrate to cover the optical waveguide. Next, a CPW (Coplanar Waveguide) electrode structure having one signal electrode and two ground electrodes is formed thereon. In addition, a fiber/fiber package are attached to the both sides of the substrate so that the optical wave goes in and out of the optical waveguide. Further, a connector/connector package is attached to the signal electrode. Under this circumstances, microwave signals are given to the signal electrode via the connector. Finally, the substrate is placed in a metal package.
In this case, loss due to higher order mode propagation becomes a problem. The thickness of the chip must be ideally reduced in the range of not greater than 0.2 mm, and further air gaps must be provided up and down the chip to reduce the microwave attenuation due to the higher order mode propagation. However, it is difficult to handle the chip with such a structure and further attach the fiber/fiber connector for such a thin chip. Therefore, the chip is arranged on the dielectric substrate, as mentioned above. However, the microwave attenuation is increased because the device has no air gaps.
To the solve the problem, suggestion has been made about a device which comprises the dielectric substrate and which has a slit structure therein for the entire portion except for edge portions to reduce the microwave attenuation by the inventors, as disclosed in U.S. Pat. No. 5,502,780. However, it is also difficult to handle the device because the dielectric substrate has the above slit structure.
On the other hand, a wider modulation bandwidth (higher than 20 GHz) and a lower driving voltage (lower than 3.5 V) are needed to realize further higher speed communication system. To this end, the microwave attenuation must further be reduced. As above mentioned, the bandwidth of the optical modulator is restricted by the microwave attenuation. It has been experimentally confirmed that the loss of the microwave mainly depends on a feeder portion of a signal electrode, a connection between a connector and the signal electrode and a connector package and the like. Generally speaking, the characteristic impedance, the velocity mismatch between the optical wave and the microwave or the difference of the effective index of refraction can be set to a desired value by optimizing electrode parameters, such as the width of the signal electrode, the gap between the signal electrode and the ground electrode, and a parameter of the buffer layer.
The stripline conductor loss as a part of the microwave attenuation of the electrode is also determined by the electrode parameters. For example, disclosure has been made about an impedance matching circuit for matching impedance between the signal electrode and the connector in Unexamined Japanese Patent Publication No. H7-98442. However, it is expected that the microwave loss is increased by the impedance matching circuit.
By the way, the width of the signal electrode generally falls within the range between 5 and 10 .mu.m, while the gap between the signal electrode and the ground electrode falls within the range between 10 and 30 .mu.m. This is given in paper "A wide-band Ti:LiNbO.sub.3 optical modulator with a novel low-microwave attenuation CPW electrode structure", Proceedings of IOOC-95, WD1-3, 1995. Further, the width of the connector generally falls within the range between 0.22 and 0.5 mm.
It is ideal that the width of the signal electrode is made constant and further equal to that of the connector so as to reduce the microwave loss. However, the width of the connector in the practical use falls within the range 0.22 and 0.5 mm, while the width of the signal electrode generally falls within the range between 5 and 10 .mu.m. Therefore, the feeder portion of the signal electrode must be widen in a taper form to connect the signal electrode with the connector. Consequently, the microwave loss is increased.